The field of the invention is electronic controllers for operating industrial equipment.
Sequence controllers have been used to advance industrial equipment through a series of programmed steps. The sequencer is connected to one or more input devices, such as limit switches, photoelectric cells, counters, pushbuttons, selector switches, etc., and in response to the operation of these input devices, the controller energizes and deenergizes operating devices on the controlled system. Such controllers typically include drum switches or diode matrices which store a sequence of programmed steps and the controller advances through the sequence one step at a time. Although some sequence controllers such as that disclosed in British Pat. Specification No. 1,126,891 include means for jumping steps in the sequence, all sequence controllers are characterized by the fact that only the selected input devices necessary to advance the sequencer to the next step are monitored at any one time.
Although they are more elaborate than sequence controllers, process controllers and programmable controllers allow control of asynchronous machines. In programmable controllers such as that disclosed in U.S. Pat. No. 3,810,118 entitled "Programmable Matrix Controller", for example, the status of all of the imput devices on the machines to be controlled are rapidly and repeatedly scanned by the controller and when conditions are proper, as determined by a program stored in the controller memory, one or more operating devices on the machines are energized or deenergized. A substantial amount of high speed logic circuitry is required in programmable controllers to provide this rapid scanning and to quickly make the logical decisions necessary to energize or deenergize the appropriate operating devices on the machines being controlled. Because of their higher cost, process controllers and programmable controllers have typically been applied to perform relatively complex control functions.